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The Automated Readability Index (ARI)

The Automated Readability Index (ARI) is a readability test designed to assess the understandability of a text. Like other popular readability formulas, the ARI formula outputs a number which approximates the grade level needed to comprehend the text. For example, if the ARI outputs the number 10, this equates to a high school student, ages 15-16 years old; a number 3 means students in 3rd grade (ages 8-9 yrs. old) should be able to comprehend the text.

** ( Use our free Automated Readability Index Calculator to grade your text using the ARI formula).

The Automated Readability Index is derived from ratios representing word difficulty (number of letters per word) and sentence difficulty (number of words per sentence). The first consideration in developing the Automated Readability Index was to establish that the factors used relate to those found in other indices. The factor relating to sentence structure (average number of words per sentence) is identical to that found in most currently used indices , such as the Coleman-Liau Index, so no verification was required. The verification of the relationship between the word structure factor was also virtually self-evident.

Most readability indices consist of two factors. One factor relates to sentence structure and is most generally a measure of the average number of words per sentence. The other factor generally relates to word structure and is usually based on either the proportion of easy words determined with reference to word list (Dale and Chall, 1948) or the average number of syllables per word (Flesch, 1951). While the word list has many advantages, especially in 4th lower grades, it is both slow and relatively inaccurate when applied to adult reading material. Syllable counts prove to be deceptively unreliable.

Here is the formula to calculate the Automated Readability Index:

Automated Readability Index

Characters are the number of letters and numbers.

The Automated Readability Index outputs a number that approximates the age needed to understand the text. As a rough guide, US grade level 1 corresponds to ages 6 to 8. Reading level grade 8 corresponds to the typical reading level of a 14 year-old US child. Grade 12, the highest US secondary school grade before college, corresponds to the reading level of a 17 year old.

Here is a breakdown of grade levels in the U.S.:
  • 5-6 yrs. old — Kindergarten
  • 6-7 yrs. old — First Grade
  • 7-8 yrs. old — Second Grade
  • 8-9 yrs. old — Third Grade
  • 9-10 yrs. old — Fourth Grade
  • 10-11 yrs. old — Fifth Grade
  • 11-12 yrs. old — Sixth Grade
  • 12-13 yrs. old — Seventh Grade
  • 13-14 yrs. old — Eighth Grade
  • 14-15 yrs. old — Ninth Grade
  • 15-16 yrs. old — Tenth Grade
  • 16-17 yrs. old — Eleventh grade
  • 17-18 yrs. old — Twelfth grade
  • 18-22 yrs. old - College
Let's use the following sample to calculate the Automated Readability Index:
The rule of rhythm in prose is not so intricate. Here, too, we write in groups, or phrases, as I prefer to call them, for the prose phrase is greatly longer and is much more nonchalantly uttered than the group in verse; so that not only is there a greater interval of continuous sound between the pauses, but, for that very reason, word is linked more readily to word by a more summary enunciation. Still, the phrase is the strict analogue of the group, and successive phrases, like successive groups, must differ openly in length and rhythm. The rule of scansion in verse is to suggest no measure but the one in hand; in prose, to suggest no measure at all. Prose must be rhythmical, and it may be as much so as you will; but it must not be metrical. It may be anything, but it must not be verse.

The Automated Readability Index of this sample text is 10.6 or 11 (always round up the final number). If we look at our grade level chart from above, the final output of 11 means high school students should be able to comprehend this text. Ages below 16 years old will find this passage difficult to read and understand.

Here are the final results of the sample text:
  • Total # of words: 151
  • Average # of words per sentence: 25
  • Total # of sentences: 6
  • Total # of characters: 623
  • Average # of characters per word: 4.1
  • Total # of words with 3+ syllables: 14 Percent of
  • 3+ syllables in text: 9%
  • Average # of syllables per word: 1
  • Total syllables in text: 206
Automated Readability Index: 10.6
Grade level needed to comprehend your text: 15-17 yrs. old (Tenth to Eleventh graders)

Unlike the other indices, the ARI, along with the Coleman-Liau, relies on a factor of characters per word, instead of the usual syllables per word. The number of characters is more readily and accurately counted by computer programs than syllables.

The readability of most written material varies considerably from passage to passage. As a result, the reliability of any readability index is limited by the length of the text sample. Many factors affect the evaluation of any result from a readability formula, especially when you consider who will be reading your text, a child or an adult reader, as well as his background in the content area. If the written material is in his area of competency, readability is less important than if your text focused on a subject matter area with which he had little knowledge. Thus, a new employee may have difficulty reading a manual that is easily read by more experienced persons. An economist may be able to read most written material dealing with his specialty, yet, have difficulty reading comparatively introductory texts in electronics. Conversely, the electronics engineer might find his first encounter with a volume on economics to be difficult reading. In many ways this is similar to learning a foreign language.

Additionally, the intent of the reader is possibly the most important factor. A person reading for recreation or general interest would probably prefer books with a relatively low readability index. The same reader searching for the solution to a specific problem of concern to him might successfully undertake the reading of a much more difficult source.

Generally, the readability of a book as determined by the Automated Readability Index can only account for a portion of the factors involved in selecting appropriate written material. The background, interests and motivation of the reader and the writing style and skill of the author are more important but beyond the scope of this formula, or any other known mathematical formula.

** ( Use our free Automated Readability Index Calculator to grade your text using the ARI formula).
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